Covid-19 Vaccine Side Effects: Your Questions Answered

Almost three million people in the United States receive the Covid-19 vaccine every day. And each new burst raises new questions about what to expect after vaccination.

Last week I asked readers to send me their questions about vaccinations. Here are some of the answers.

Short-lived side effects such as fatigue, headache, muscle pain, and fever are more common after the second dose of the Pfizer BioNTech and Moderna vaccines, which each require two shots. (The Johnson & Johnson vaccine only requires a single shot.) Patients who experience uncomfortable side effects after the second dose often describe feeling like they have a bad flu and use phrases like “it blew me out” or ” I was useless for two days. “During vaccine studies, patients were advised to take a few days off after the second dose, just in case they had to spend a day or two in bed.

The data collected by v-safe, the app that anyone can use to track side effects after vaccination, also shows an increase in reported side effects after the second dose. For example, about 29 percent of people reported fatigue after the first Pfizer BioNTech shot, but that increased to 50 percent after the second dose. The muscle pain increased from 17 percent after the first shot to 42 percent after the second. While only about 7 percent of people had chills and a fever after the first dose, that number rose to about 26 percent after the second dose.

The New York Times interviewed several dozen of the people who were newly vaccinated in the following days. They reported a wide range of reactions, from no reaction to symptoms such as uncontrolled tremors and “brain fog”. While these experiences are not pleasant, they are a sign that your own immune system is having a strong response to the vaccine.

An analysis of the safety data of the first 13.7 million Covid-19 vaccine doses to Americans found that side effects were more common in women. While severe reactions to the Covid vaccine are rare, almost all cases of anaphylaxis or life-threatening allergic reactions have occurred in women.

The finding that women are more likely to report and experience unpleasant side effects from the Covid vaccine is consistent with other vaccines as well. Women and girls after flu vaccinations and vaccines against measles, mumps and rubella (MMR) and hepatitis A and B can produce up to twice as many antibodies. One study found that in nearly three decades, women accounted for 80 percent of all anaphylactic diseases in adults.

Although women report side effects more often than men, the higher rate of side effects in women also has a biological explanation. Estrogen can stimulate an immune response, while testosterone can weaken it. In addition, there are many immune-related genes on the X chromosome, of which women have two copies and men only one. These robust immune responses explain why 80 percent of autoimmune diseases affect women. You can read more about women and vaccine side effects here.

Side effects get all the attention, but when you look at data from vaccine clinical trials and the real world, you will find that many people don’t experience side effects beyond an aching arm. In the Pfizer vaccine studies, about one in four patients reported no side effects. In the Moderna studies, 57 percent of patients (64 or younger) reported side effects after the first dose – these jumped to 82 percent after the second dose, meaning that almost one in five patients reported no reaction after the second shot.

A lack of side effects doesn’t mean the vaccine isn’t working, said Dr. Paul Offit, professor at the University of Pennsylvania and a member of the Food and Drug Administration’s Vaccine Advisory Board. Dr. Offit found that a significant number of people reported no side effects during the vaccine trials, and yet the studies showed that around 95 percent of people were protected. “That proves you don’t have to have side effects to be protected,” he said.

Nobody really knows why some people have a lot of side effects and others don’t. We know that younger people develop a stronger immune response to vaccines than older people, whose immune systems become weaker as they get older. Women usually have stronger immune responses than men. But even these differences don’t mean you aren’t protected if you don’t feel a lot after the shot.

Scientists still aren’t sure how effective the vaccines are in people whose immune systems may be weakened by certain conditions, such as cancer treatments, HIV infection, or because they are taking immunosuppressive drugs. However, most experts believe that the vaccines still offer these patients some protection against Covid-19.

Updated

April 1, 2021, 11:02 p.m. ET

The bottom line is that although individual immune responses can vary, the data collected so far shows that all three US-approved vaccines – Pfizer-BioNTech, Moderna, and Johnson & Johnson – are effective against serious illness and death from Covid-19 .

You shouldn’t try to prevent discomfort by taking a pain reliever before receiving the shot. The concern is that premedication with a pain reliever like acetaminophen (Tylenol) or ibuprofen (Advil, Motrin), which can prevent side effects like arm pain, as well as fever or headache, could also weaken your body’s immune response.

While it’s possible that taking a pain reliever before your shots may have dampened your body’s immune response, vaccine experts say you shouldn’t worry and shouldn’t try to get another shot. Studies of other vaccines suggest that while premedication can reduce the body’s immune response to a vaccine, your immune system can build strong enough defenses to fight infection. A review of studies involving more than 5,000 children compared antibody levels in children who took pain relievers before and after vaccinations and in children who did not. They found that pain medication had no significant effect on the immune response and that children in both groups produced adequate levels of antibodies after their shots.

The high effectiveness of all Covid vaccines suggests that even if taking Tylenol before the shot weakens your body’s immune response, there is some margin and you are likely still well protected against Covid-19. “You should feel reassured that you have enough immune response to be protected, especially with vaccines that are this good,” said Dr. Offit.

“It’s okay to treat side effects with pain relievers,” said Dr. Offit, but if you don’t really need one, “don’t take it”.

While most experts believe it is safe to take a pain reliever to relieve post-vaccination discomfort, they do not recommend taking it preventively after the shot or if your symptoms are manageable without the drug. The concern about taking an unnecessary pain reliever is that it may weaken some of the effects of the vaccine. (In terms of vaccine, there is no significant difference if you choose acetaminophen or ibuprofen.)

During the Moderna study, about 26 percent of people took acetaminophen to reduce side effects, and the vaccine’s overall effectiveness was still 94 percent.

Research and individual reports suggest that people with a previously diagnosed Covid-19 infection may react more strongly and experience more side effects after their first dose of vaccine than people who have never been infected with the virus. A strong reaction to your first dose of vaccine could also be a sign that you were previously infected, even if you weren’t aware of it.

If you’ve previously tested positive for Covid-19 or had a positive antibody blood test, be prepared for a stronger reaction to your first dose and plan a few days off just in case. Not only will it be more convenient to stay at home and rest in bed, the vaccine side effects may be similar to symptoms of Covid-19, and your staff won’t want to be around you anyway.

Studies suggest that a dose might be appropriate for people with a previously confirmed case of Covid-19, but medical guidelines have not changed so far. If you received the Pfizer-BioNTech or Moderna vaccines, you should plan to receive your second dose, even if you have had Covid-19. Skipping your second dose can cause problems if your employer or airline requests proof of vaccination in the future. If you live in an area where Johnson & Johnson’s single-dose vaccine is available, you can be fully vaccinated after just one dose. Read more about the vaccine response in people with Covid-19 here.

The vaccines appear to be effective against a new variant that originated in the UK and is rapidly dominating the US. However, some variants of the coronavirus, especially one first identified in South Africa and one in Brazil, appear to be better able to evade antibodies in vaccinated individuals.

That sounds worrying, but there is reason to be hopeful. Vaccinated individuals exposed to a more resistant variant still appear to be protected from serious diseases. And scientists have a sufficiently clear understanding of the variants that they are already working on to develop booster shots that target the variants. The variants identified in South Africa and Brazil are not yet widespread in the United States.

People who are vaccinated should continue to wear masks in public and follow public health guidelines, but they shouldn’t live in fear of variations, said Dr. Peter J. Hotez, Dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston. “If you are vaccinated you should feel pretty sure how protected you are,” said Dr. Hotez. “It is unlikely that you will ever go to a hospital or intensive care unit with Covid-19. In time, you will see a recommendation for a booster. “

I hope these answers will reassure you about your own vaccine experience. For a more complete list of questions and answers, see our dedicated vaccine tool, “Answers to All Your Covid-19 Vaccination Questions”.

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